This invention relates to a process and catalyst for reducing the aromatics and olefins content of hydrocarbon distillate products. More particularly, this process relates to an improved catalytic hydrogenation process and catalyst wherein the catalyst comprises platinum and palladium incorporated onto a mordenite support.
For the purpose of the present invention, the term "hydrogenation" is intended to be synonymous with the terms "hydrotreating" and "hydroprocessing," and involves the conversion of hydrocarbons at operating conditions selected to effect a chemical consumption of hydrogen. Included within the processes intended to be encompassed by the term hydrogenation are aromatic hydrogenation, dearomatization, ring-opening, hydrorefining (for nitrogen removal and olefin saturation), and desulfurization (often included in hydrorefining). These processes are all hydrogen-consuming and generally exothermic in nature. For the purpose of the present invention, distillate hydrogenation does not include distillate hydrocracking which is defined as a process wherein at least 15% by weight of the distillate feedstock boiling above 430.degree. F. is converted to products boiling below 430.degree. F.
Petroleum refiners are now facing the scenario of providing distillate fuels, boiling in the range of from about 150.degree. F. to about 700.degree. F., with substantially reduced sulfur and aromatics contents. Sulfur removal is relatively well defined, and at constant pressure and adequate hydrogen supply, is generally a function of catalyst and temperature.
Aromatics removal presents a substantially more difficult challenge. Aromatics removal is generally a function of pressure, temperature, catalyst, and the interaction of these variables on the chemistry and thermodynamic equilibria of the dearomatization reaction. The dearomatization process is further complicated by the wide variances in the aromatics content of the various distillate component streams comprising the hydrogenation process feedstock, the dynamic nature of the flowrates of the various distillate component streams, and the particular mix of mono-aromatics and polycyclic aromatics comprising the distillate component streams.
The criteria for measuring aromatics compliance can pose additional obstacles to aromatics removal processes. The test for measuring aromatics compliance can be, in some regions, the FIA aromatics test (ASTM D1319), which classifies mono-aromatics and polycyclic aromatics equally as "aromatics." Hydrogenation to mono-aromatics is substantially less difficult than saturation of the final ring due to the resonance stabilization of the mono-aromatic ring. Due to these compliance requirements, hydrogenation to mono-aromatics is inadequate. Dearomatization objectives may not be met until a sufficient amount of the polycyclic aromatics and mono-aromatics are fully converted to saturated hydrocarbons.
While dearomatization can require a considerable capital investment on the part of most refiners, dearomatization can provide ancillary benefits. Distillate aromatics content is inextricably related to cetane number, the accepted measure of diesel fuel quality. The cetane number is highly dependent on the paraffinicity of molecular structures, whether they are straight-chain or alkyl attachments to rings. A distillate stream which comprises mostly aromatic rings with few or no alkyl-side chains generally is of lower cetane quality material while a highly paraffinic stream is generally of higher cetane quality.
Dearomatization of refinery distillate streams can increase the volume yield of distillate products. Aromatic distillate components are generally lower in gravity than their similarly boiling paraffinic counterparts. Saturation of aromatic rings can convert these lower API gravity aromatic components to higher API gravity saturated components and expand the volume yield of distillate product.
Dearomatization of refinery distillate streams can also provide increased desulfurization and denitrogenation beyond ordinary levels attendant to distillate hydrogenation processes. Processes for the dearomatization of refinery distillate streams can comprise the construction of a new dearomatization facility, the addition of a second-stage dearomatization step to an existing distillate hydrogenation facility, or other processing options upstream of distillate hydrogenation or at the hydrogenation facility proper. These dearomatization steps can further reduce the nitrogen and sulfur concentrations of the distillate component and product streams, thus reducing desulfurization and denitrogenation catalyst and temperature requirements in existing distillate hydrogenation facilities designed primarily for hydrorefining. Reduced distillate sulfur and nitrogen concentrations can additionally increase the value of these streams for use as blending stocks to sulfur-constrained liquid fuel systems and as fluid catalytic cracking unit (FCC) feed.
While distillate dearomatization can provide cetane number improvement, volume expansion, and additional desulfurization and denitogenation, the process has seldom been attractive in view of the large capital costs and the fact that many refiners have not reached distillate cetane limitations. Now that legislation exists and further legislation is being considered to mandate substantial reductions in distillate aromatics content, the demand for distillate dearomatization processes is now being largely determined by the incentive to continue marketing distillates.
Hydrogenation processes and catalysts for the treatment of distillate streams has been the subject of several patents. U.S. Pat. Nos. 3,736,252, 3,773,654, 3,969,222, 4,014,783, 4,070,272, 4,202,753, 4,610,779, and 4,960,505 are all directed towards processes for hydrogenating and dearomatizing distillate fuels.
The use of mordenite in catalyst supports for hydrogenation has met with limited success and is particularly rare in distillate dearomatization. Mordenite, and zeolite supports in general, have not been commonly used in hydrogenation processes because the silica content, in combination with common commercial hydrogenation metals, such as nickel, molybdenum, and cobalt, can provide lower desulfurization activity, have a tendency to promote undesired cracking reactions, and can be prone to early deactivation.
U.S. Pat. No. 3,197,398 to Young discloses a distillate and gas oil hydrocracking process using a catalyst comprising a group VIII metal (IUPAC) such as palladium on a crystalline alumino-silicate support such as faujasite or mordenite having a silica to alumina molar ratio between about 2.5 and 10 (correlating to a silicon to aluminum atomic ratio of between about 1.25 and 5). The hydrocracking process and catalyst are designed to convert high-boiling mineral oil feedstocks to lower boiling products such as gasoline. Hydrocracking reactions are not desired in the hydrogenation process and catalyst of the present invention because hydrocracking reduces liquid product yield, increases undesirable light gas make, increases catalyst deactivation rates, and reduces distillate product cetane numbers.
S. M. Kovach and R. A. Kmecak, in a paper entitled "Hydrogenation of Aromatics in the Presence of Sulfur," presented before the Division of Petroleum Chemistry Inc., American Chemical Society, in Houston on Mar. 23-28, 1980, further illustrate the resistance in the art to teach or suggest use of a hydrogenation catalyst comprising hydrogenation metals on a mordenite support for distillate hydrogenation. Kovach and Kmecak teach that palladium on a mordenite support in hydrogenation service readily deactivates, provides poor desulfurization, and exhibits dehydrogenation activity. The catalysts were shown to only tolerate feedstocks having less than 50 ppm sulfur.
The use of metal mixtures on a catalyst support has also been the subject of extensive research. (See P. N. Rylander, Catalytic Hydrogenation over Platinum Metals, Academic Press, New York 1967.) Two platinum metal catalysts, when used together, can give better rates or better yields than either catalyst individually. However, except for certain selected examples, there seems to be no way of predicting when mixtures of catalysts will prove advantageous. A useful guide as to the probable effectiveness of coprecipitated metal catalysts, is the performance of a mechanical mixture of the two metals. (See Rylander, at pages 9-11.)
U.S. Pat. No. 3,943,053 to Kovach et al. discloses a hydrogenation process using a catalyst comprising platinum and palladium on an inert oxide support such as beta, eta, or gamma alumina. The process provides distillate hydrogenation, but with limited dearomatization activity.
It has surprisingly been found that processes having a catalyst incorporating metal mixtures of platinum and palladium onto a support comprising mordenite, result in substantially improved hydrogenation compared to prior art hydrogenation processes including processes having a catalyst incorporating platinum and palladium on inert oxide supports such as alumina. This particular synergy is more profound (in contradistinction to the teachings of Rylander) since physical mixtures of platinum and palladium on a mordenite support have been shown not to provide improved hydrogenation.
It is therefore an object of the present invention to provide a process and catalyst that provide improved distillate dearomatization.
It is an object of the present invention to provide a process and catalyst that provide improved distillate desulfurization and denitrogenation.
It is an object of the present invention to provide a process and catalyst that increase distillate cetane number.
It is an object of the present invention to provide a process and catalyst that expand the volume of the distillate feedstock.
It is yet another object of the present invention to provide a catalyst that has superior crush strength and durability.
Other objects appear herein.